This invention is concerned with a feeder mechanism for supplying gobs of plastic material e.g. molten glass, the mechanism comprising a plunger driving means operable to move a plunger into and out of an opening in the bottom of a channel containing plastic material so that, when the plunger is out of the opening, the plastic material flows out through the opening, and apparatus for shearing gobs from the material flowing out through the opening, the apparatus comprising a set of shear blades movable towards and away from one another between open and shearing positions thereof, and a driving system arranged to drive the shear blades.
In the operation of a glass container forming machine, a feeder mechanism as described above supplies gobs of molten glass which are moulded into containers by the machine. It is necessary that the shear blades of the mechanism perform a precise stroke between an overlapping shearing position and an open position thereof. The length of the stroke is constant for all working conditions of the feeder mechanism but it is necessary to vary the speed and relative timings of different portions of the stroke, when changing from gobs of one size to gobs of another size or when changing the shape of the gob, since there is a different desired position against time curve for the blades for each size and shape of gob.
In order to achieve the desired position against time curve for a particular gob, present feeder mechanisms each comprise a driving system for the shear blades which comprises a plate cam having an edge profile in accordance with the desired position against time curve for the blades, a cam follower running on the edge of the cam being mechanically connected to the blades. The cam is rotated continuously in the operation of the machine at a constant speed so that successive strokes of the shear blades are made with the position against time curve of the shear blades being determined entirely by the shape of the cam. When a gob of a different size or shape is to be produced, the cam is removed and replaced by another having a different profile. It is usually necessary to maintain a supply of alternative cams. Furthermore, it is not possible to vary the position against time curve without producing a new cam and stopping the machine to fit it.
It is an object of the present invention to provide a feeder mechanism in which the position against time curve of the shear blades can be varied without requiring a new cam and without requiring the machine to be stopped.